The present invention relates to electrolysis cells for producing aluminum comprising a cathode and an anode of the continuous type. The anode is composed of blocks of carbon glued or mechanically attached to one another, and new blocks of carbon are attached to the anode to replace carbon material consumed during the electrolysis process.
Aluminum is nowadays produced in electrolysis cells based on two different principles, namely cells provided with self-baking anodes, so called Soederberg anodes, and cells equipped with prebaked carbon anodes which have to be exchanged with new anodes due to their consumption during the electrolysis process.
Electrolysis cells with anodes of the prebaked type have the advantage that the voltage drop is less than for the Soederberg type. This mainly has to do with the fact that the specific electrical resistance in prebaked anodes is lower than the resistance in the coke mass of the Soederberg anodes. In addition, the drop in voltage between the current conductors and the carbon material is lower for the prebaked anodes than for the Soederberg anodes, as the current conductors for the prebaked anodes are connected to the carbon blocks beforehand and can be firmly connected by means of gluing, screwing, casting or the like, whereas the current conductors for the Soederberg anodes are placed in position in the carbonatious anode mass during the electrolysis process in such a way that when they have reached their lowermost position they can be pulled up to be repositioned, such that the resulting connection is relatively loose.
On the other hand, the prebaked anodes of the discontinuous type are encumbered with several disadvantages. As they have to be replaced by new ones before they are completely used, there is an anode rest loss of about 15-25% of the total anode consumption. Further, the exchange and maintenance work is extensive and results in great expense.
Due to the disadvantage of the traditional electrolysis cells, the aluminum production companies have been engaged in research and development to provide electrolysis cells with prebaked anodes of the continuous type as mentioned initially. Norwegian Patent No. 98126 discloses a cell for producing aluminum in which is used a continuous prebaked anode composed of blocks of carbon being attached to one another by means of gluing. Except that the anode is composed of glued blocks of carbon, the solution according to this reference is based on the Soederberg principle as the anode is disposed in a vertically sliding relation with a steel jacket, and the electric current is conducted via contact bolts provided in holes in the top side of the anode. When attaching new carbon blocks to the anode, the bolts have to be pulled out. This is impractical and time consuming and results in high operation costs. This solution has, therefore, not found any practical application.
In Norwegian Patent No. 98126, there is further shown a cell for producing aluminum where two electrodes are arranged in side by side relation in steel jackets. The electrodes are composed of blocks of carbon onto which can be joined new blocks of carbon as the anodes are used. The feeding of the anodes is accomplished by means of jacks provided on top of the steel jackets. Further, for the supply of electric current to the electrodes and to provide the necessary friction to hold the electrodes, the lower ends of the steel jackets are provided with pressure devices in the form of weight arms, each acting on an exchangeable sliding contact which is influenced by a spring provided with an individual screw adjusting means.
A disadvantage with the above solution is that the pressure devices, which are complicated in their structural design, comprising screws and moveable parts, are disposed slightly above the electrolytic bath and will, therefore, very quickly become damaged by the heat and harmful gases from the bath. It is further a disadvantage that the pressure devices are large in size, as this reduces the effective anode area and makes it more difficult to get access to the electrolytic bath, for instance, in connection with taping of metal, crust breaking, etc.
Since only two electrodes having large carbon anodes which are cumbersome to handle are used, the maintenance of such cells is difficult. Furthermore, the large carbon blocks for which their is a relatively long distance between current connectors, provide an ineffective current supply and uneven current distribution in the anode.
Due to the above disadvantages, the solution disclosed in Norwegian Patent No. 73535 has not found any practical use.